在《清洁空气法案》强制要求美国使用无铅气体的二十年里，铅在世界范围内的累积量显著下降。
In the twenty years that the Clean Air Act has mandated unleaded gas use in the United States, the lead accumulation worldwide has decreased significantly.
研究人员说，患癌症和支气管炎的人数的增加可能反映了吸烟习惯的改变和空气质量的下降。
An increase in some cancers and bronchitis may reflect changing smoking habits and poorer air quality, say the researchers.
空气质量整体下降了。
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